One of the things that is really irritating is the fact that drivers that use contig-dma sometimes want to support USERPTR, allowing applications to pass pointers to the driver that point to physically contiguous memory that was somehow obtained, and that userspace has no way of knowing whether the driver has this requirement or not. A related issue is that, depending on the DMA engine, the user pointer might have some alignment requirements (page aligned, or at minimum 16 bytes aligned) that userspace has no way of knowing. The same alignment issue is present also for dma-buf. I propose to take one reserved field from struct v4l2_create_buffers and from struct v4l2_requestbuffers and change it to this: __u32 flags; #define V4L2_REQBUFS_FL_ALIGNMENT_MSK 0x3f #define V4L2_REQBUFS_FL_PHYS_CONTIG (1 << 31) Where the alignment is a power of 2 (and if 0 the alignment is unknown). The max is 2^63, which should be enough for the foreseeable future :-) If the physically contiguous flag is set, then the buffer must be physically contiguous. These flags can be set for every vb2 dma implementation: dma-contig: the PHYS_CONTIG flag is always set and the alignment is (unless overridden by the driver) page aligned. dma-sg: the PHYS_CONTIG flag is 0 and the alignment will depend on the driver DMA implementation. Note: malloc will align the buffer to 8 bytes on a 32 bit OS and 16 bytes on a 64 bit OS. vmalloc: PHYS_CONFIG is 0 and the alignment should be 3 (2^3 == 8) for 32 bit and 4 (2^4=16) for 64 bit OS. This matches malloc() which will align the buffer to 8 bytes on a 32 bit OS and 16 bytes on a 64 bit OS. The flags field can be extended with things like OPAQUE if the buffers cannot be mmapped (since they are in protected memory). Comments? Alternative solutions? Regards, Hans -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
